1. Field of the Invention
The present invention relates to a crystal pulling method employing, for example, the Czochralski method (the CZ method), and particularly to an improvement on a technique for mechanically holding a heavy single crystal in order to pull the crystal safely and reliably, which improvement reduces variations in the diameter of a growing crystal at a point of time when the crystal is held, thereby enabling accurate growth of the crystal.
2. Description of the Related Art
Regarding a method of pulling a single crystal from a melt of a semiconductor material, such as silicon, contained in a crucible, the present inventors have proposed an improved pulling method as described in, for example, Japanese Patent Application No. 8-212055.
As shown in FIG. 4, in the pulling method, a seed crystal 53 held by a seed chuck 52 at the lower end of a wire 51 is brought into contact with material melt contained in an unillustrated crucible, and the seed crystal 53 is pulled by a seed chuck lifting mechanism 54 in order to grow a single crystal C below the seed crystal 53. In order to enable safe growth of the single crystal C, the pulling method employs an apparatus having the following structure: a retaining case 57 is mounted on a slider 56 which is vertically moved by an unillustrated slider moving mechanism; the seed chuck lifting mechanism 54 for winding the wire 51 is provided within the retaining case 57; and a lifting jig 60 composed of paired members is provided below the retaining case 57. The lower ends of the paired members of the lifting jig 60 are opened and closed through operation of cylinder units 58. At the beginning of pulling, the single crystal C is pulled by the seed chuck 52, and pulling by the seed chuck 52 is switched to pulling by the lifting jig 60 when the single crystal C grows and becomes heavy.
That is, initially, with the slider 56 fixed at a certain position, the seed crystal 53 held by the seed chuck 52 is brought into contact with the material melt, and subsequently the seed chuck 52 is pulled by the seed chuck lifting mechanism 54. As a result, as shown in FIG. 4A, a neck portion 59, a corrugated portion Ck, and the single crystal C are formed below the seed crystal 53.
When the corrugated portion Ck rises to a predetermined position shown in FIG. 4B, the lifting jig 60 is operated as shown in FIG. 4C such that the lower end portions of the paired members of the lifting jig 60 approach the smaller-diameter portion of the corrugated portion Ck with a clearance remaining therebetween. Subsequently, as shown in FIG. 4D, the slider 56 is raised, while the seed chuck lifting mechanism 54 is operated in reverse in order to lower the single crystal C relative to the slider 56, such that the larger-diameter portion of the corrugated portion Ck comes into contact with and sits on the lifting jig 60.
At this time, as shown in FIG. 5, control is performed such that the total speed Vt=Vb+Va (where Vb represents the rising speed of the slider 56 and Va represents the speed of the seed chuck 52 relative to the slider 56) becomes constant. It is to be noted that when the seed chuck 52 moves downward relative to the slider 56, the value Va becomes negative. During the second half of the control, the corrugated portion Ck contacts the lifting jig 60 at, for example, point x, and a predetermined portion of the entire load of the crystal is shifted to the lifting jig 60 before point y, and the seed chuck 52 is stopped at the point y. With this operation, pulling by the seed chuck lifting mechanism 54 is switched to pulling by the lifting jig 60 (4E).
In the above-described technique, since the total speed Vt=Vb+Va is controlled so as to be constant when the pulling means is switched from the seed chuck to the lifting jig, the actual pulling speed (growth rate) of a crystal is constantly maintained at a desired pulling speed V. This control is effective for growing a crystal while guaranteeing high accuracy of, for example, the diameter of the crystal. However, through further experimental studies, the inventors of the present invention have found that accuracy of a crystal, particularly the accuracy of the diameter of the crystal, can be improved further if the control conditions are changed at a point (point x) in time when the shifting of the load to the lifting jig is started.